Impement `Font` API Object

We want to expose font loading to users. Bevy uses cosmic-text at the moment for font assets although may switch to parley soon. For the most part, this will involve just loading a Bevy Font asset, especially until we have a more solid path for UI. But, we'll also want to expose facilities for users to get curve information back for a given font, see https://docs.rs/cosmic-text/latest/cosmic_text/struct.SwashCache.html.